1. Field
The present disclosure relates to a method and apparatus for assembly of device, integrated circuit, and/or passive components on a substrate to provide hybrid electronic, optoelectronic, or other types of integrated systems. For example, the present disclosure describes a method and apparatus for transporting and dispersal of microstructures by fluidic self-assembly onto a substrate wafer.
2. Description of Related Art
Increasingly complex integrated electronic and optoelectronic systems require larger numbers of integrated circuits and devices to implement increasingly complex system functions. However, to achieve cost and weight goals, it is preferred that these integrated systems be implemented with as few separate device structures as possible. One approach is to fabricate all of the integrated circuits and devices on a single wafer or portion of a wafer, which provides the structural base for the system and minimizes the interconnect distances between circuits and devices. Such fabrication may be referred to as “wafer-scale” integration.
Many complex integrated electronic and optoelectronic systems require the use of integrated circuits and devices that utilize different semiconductor technologies. One approach known in the art for wafer-scale integration of different semiconductor technologies is heteroepitaxy. The heteroepitaxy approach may limit the number of different devices and material systems that can be successfully integrated. Moreover, growth and fabrication procedures optimized for a single device technology often must be compromised to accommodate dissimilar material systems. Finally, testing of individual portions of the integrated system may be made difficult by the fabrication techniques used to accommodate dissimilar material systems on a single wafer.
Since it may be difficult to fabricate high performance systems with multiple types of devices using heteroepitaxy approaches, it may be preferable to fabricate separate arrays of devices or circuit modules and couple these separately fabricated components to a host wafer. This approach allows each individual component to have state-of-the-art performance and high yield (due to pre-testing). Each component may use proven device and circuit architectures, while optimum epitaxial growth and/or device processing sequences are employed to fabricate each component.
The separate components may be individually integrated with the host wafer using any one of several established methods for chip-level integration. These methods generally rely upon surface-mounting techniques for attaching complete die assemblies using solder bumps or wire bonding. The most advanced of these methods is the “flip-chip” technique that can support integration of a wide variety of device technologies and fully utilizes the costly, high-performance device wafer real estate. However, flip-chip is generally limited to relatively large size components, typically greater than 1 square millimeter, and is inefficient for the placement of large numbers of components due to its serial nature.
Pick-and-place assembly techniques for positioning components with sizes less than a millimeter on a substrate are known in the art. See, for example, Saitou et al., “Externally Resonated Linear Microvibromotor for Microassembly,” J. Microelectromech. Syst., vol. 9, pp. 336–346, September 2000. However, these techniques are known to suffer limitations due to the surface adhesion forces based on the extremely small size of the components. Further, these techniques are also inefficient for the placement of large numbers of components, again due to the serial nature of the techniques.
At the wafer-scale level, self-assembly methods generally provide the best capability to allow integration of arbitrary configurations and densities of components. The most advanced of the self-assembly methods use a fluid medium to transport components to a host substrate or wafer for assembly. Two different fluidic self-assembly methods are known in the art, which differ in the underlying mechanism used to locate, position, and connect the components on the host substrate or wafer.
The first method of fluidic self-assembly uses gravitational forces and geometrical constraints to integrate components with a host substrate. The components are fabricated with specific shapes and complementary shaped receptacles are formed on the substrate for receiving the shaped components. The components are typically formed using semiconductor fabrication techniques and the receptacles are formed by using wet or dry etching techniques. A solvent such as water or ethanol is used to transport the individual components to the host substrate with the receptacles. The receptacles trap the components, which come to rest in predictable orientations due to their specific shapes. The driving potential is primarily gravitational in origin, but the fluid and surface forces may also play a role in the assembly process.
The second method of fluidic self-assembly utilizes chemically-based driving forces to govern the assembly process, where the attraction, positioning, orientation, and ordering of components is controlled by molecular interactions at the surfaces of the components and the host substrate. Molecular-based self-assembly techniques generally use surface coatings that consist of chemically-bonded films which are either hydrophobic or hydrophilic by nature. Thermodynamic driving forces control the assembly of complex arrays of components by minimizing the surface energies of the components and host substrate.
Both methods may be used together to provide for integration of electronic and opto-electronic devices into hybrid electronic systems. See, for example, A. Terfort, et al., “Self-Assembly of an Operating Electrical Circuit Based on Shape Complementarity and the Hydrophobic Effect,” Adv. Material, 10, No. 6, 1998, pp. 470–473. See also A. Terfort, et al., “Three-dimensional Self-Assembly of Millimeter-scale Components,” Nature, Vol. 386, Mar. 13, 1997, pp. 162–164.
Various apparatus and methods are known in the art for assembling microstructures onto a substrate through fluid transport. For example, U.S. Pat. No. 5,904,545, issued on May 18, 1999, to Smith et al. describes an apparatus used for fabricating electronic systems using fluidic self-assembly methods. A schematic of the apparatus is depicted in FIG. 1.
The Smith apparatus, as shown in FIG. 1, consists of a vessel that contains the substrate that is to receive the microstructures, a fluid medium with the microstructures therein, and a pumping system. The pumping system uses gas bubbles to circulate the fluid and microstructures throughout the system. A funnel shaped drain collects and concentrates the microstructures that have not been assembled onto the substrate, and directs them for re-circulation to a column where bubbles are injected. The bubbles push the fluid and microstructures up the return line and the fluid containing the microstructures is then re-dispersed over the substrate through the spout. Changing the gas flow into the return line controls the pump rates.
However, problems associated with assembling microstructures using the Smith apparatus are reported in the Ph.D. thesis of Mark Hadley (University of California—Berkeley, 1994). In that thesis, it is disclosed that tests were performed using large (1.2 mm×1.0 mm×0.235 mm) and small (150 micron×150 micron×35 micron) microstructures with substrates having complementary shaped receptacle holes. Assembly tests were performed with the Smith apparatus using 500 large Si blocks with a substrate having 191 receptacles. Tests were also performed with 30,000 small Si blocks with a substrate having ˜4096 receptacles. When using water as the transport fluid, bubbles were found to attach to the Si blocks causing them to float. For the larger blocks, the addition of a surfactant, which altered the surface properties of the microstructures, was found to stop the attachment of the bubbles. However, smaller microstructures could not escape the forces at the water/air interface. Assembly of small microstructures required non-aqueous media such as ethanol or methanol. The results disclosed in the Hadley thesis were based on the use of a gravity-based process that employed shape matching between the microstructure blocks and the substrate receptacles.
As briefly mentioned above, other procedures for assembling microstructures do not rely on gravity to aid the placement of the microstructures on the substrate. Some procedures employ selective surface coatings (i.e., rendering surfaces hydrophobic or hydrophilic) on the microstructures and/or the substrate to guide the placement. See, for example, Gracias, et al., “Forming Electrical Networks in Three Dimensions by Self-Assembly,” Science, Vol. 289, Aug. 18, 2000, pp. 1170–1172. Other procedures employ long-range forces, such as electro-static attraction, for placement. Generally, these procedures require water as the transport medium and cannot tolerate air/water interfaces (i.e., no bubbles). These interfaces are extremely high-energy surfaces that strongly attract microstructures, which leads to clumping or trapping of the coated microstructures. This behavior decreases the efficiency of the location process.
Another procedure for assembling microstructures on a substrate involves applying the microstructures in a fluid medium through the use of a pipette. See, for example, Srinivasan, et al., “Microstructure to Substrate Self-Assembly Using Capillary Forces,” J. Microelectromech. Syst., vol. 10, pp. 17–24, March 2001. Those skilled in the art will understand that this procedure does not lend itself to a manufacturing environment in which large numbers of microstructures are to be assembled on multiple substrate wafers.
Therefore, there exists a need in the art for a method and apparatus that facilitates the transport and dispersal of microstructures onto a substrate for fluidic self-assembly. There exists a further need for a method and apparatus that allows for the microstructures to be repeatedly and at least somewhat uniformly dispersed over a substrate wafer. There exists a further need for a method and apparatus that eliminates or, at least, minimizes air/fluid interfaces to avoid the attractive forces between such interfaces and small-sized microstructures. Finally, there exists a need for a method and apparatus for fluidic assembly that may support the assembly of large numbers of microstructures on multiple substrates in a manufacturing environment.